Thanks, but that post has a lot of conflicting information in it. For example specifying that SDH is for hard of hearing, but then later saying CC is for hard of hearing. It also says CC doesn’t work over HDMI which is not the case as my Apple TV is connected via HDMI and both English SDH and English CC work.
It's conflicting in the sense that the article describes both historical meanings and current meanings without always making clear what refers to what. As the article says, the terms are a tad overloaded with different meanings as well.
But CC and SDH are for hard of hearing or deaf watchers. Historically speaking, in North America mostly, the differentiation was that SDH was burnt into the footage and CC was delivered by a separate channel and the display of the CC content was not in the video file itself, but performed by the viewing equipment.
Plugging an old video player into an old TV that supports both HDMI and composite cables might do nothing when you enable CC as the channel for the CC content is not received by the TV and it may have been its responsibility to compose the signals together. However, with newer video players they may compose the signal before sending it over HDMI to the TV solving that problem.
For a service like AppleTV+ I believe that the CC option gives CC-content to tvOS for it to compose and display and SDH composes the content in the TV app, by the content providers. I am not entirely certain though, as the article says, usage has gotten a tad muddy over the years as technology advanced and terminology didn't get updated universally among all users of the terms.
